The present invention relates to housings for planar lightguide circuits and, more particularly, to a thermal housing having temperature control for maintaining a planar lightguide circuit within a fixed temperature range.
Optical wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) have gradually become the standard backbone networks for fiber optic transmission systems. WDM and DWDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM and DWDM technology.
WDM optical transmission systems employ a variety of different passive components. Such components are increasingly being fabricated on Planar Light-Guide Circuits (PLC). A planar lightguide circuit, also known as an optical integrated circuit, can be readily mass produced because the processing steps are compatible with those used in silicon integrated circuit (IC) technology, which are well known and geared for mass production. One common type of planar lightguide circuit employs doped-silica waveguides fabricated with silicon optical bench technology. Doped-silica waveguides are usually preferred because they have a number of attractive properties including low cost, low loss, low birefringence, stability, and compatibility for coupling to fiber. Such a planar lightguide circuit is fabricated on a carrier substrate, which typically comprises silicon or silica. The substrate serves as a mechanical support for the otherwise fragile lightguide circuit and it can, if desired, also play the role of the bottom portion of the cladding. In addition, it can serve as a fixture to which input and output fibers are attached so as to optically couple cores of an input/output fiber to the cores of the planar lightguide circuit. The fabrication process begins by depositing a base or lower cladding layer of low index silica on the carrier substrate (assuming the substrate itself is not used as the cladding layer). A layer of doped glass with a high refractive index, i.e., the core layer, is then deposited on top of the lower cladding layer. The core layer is subsequently patterned or sculpted into structures required by the optical circuits using photo-lithographic techniques similar to those used in integrated circuit fabrication. Lastly, a top cladding layer is deposited to cover the patterned waveguide core.
One important passive component that can be fabricated on a PLC is an arrayed waveguide grating (AWG) in which two multiport couplers are interconnected by an array of waveguides. AWGs have a variety of different uses and may serve, for example, as multiplexers, demultiplexers and static routers.
One of the problems arising from the use of some planar lightguide circuits such as an AWG is their sensitivity to temperature changes, and to physical stresses that impair their reliability. For example, in an AWG, because the operating wavelengths of the several individual channels differ by such a small degree, any expansion or contraction or bending due to temperature fluctuations will degrade the optical performance and, in the extreme, cause circuit failure. Likewise, temperature fluctuations less than 1xc2x0 C. may cause degradation or failure. It has been found that degradation or failure can generally be prevented and reliability of the circuit insured if the temperature of the device is maintained at a predetermined temperature in a range of 75xc2x0 C. to 90xc2x0 C. This maintenance temperature, specific to the individual circuit, must be controlled to within a few degrees Celsius even though the ambient temperature may vary from, for example, 0xc2x0 C. to 70xc2x0 C. Thus, some sort of protective housing must be provided for the planar lightguide circuit.
Housings for maintaining optical components at a constant temperature are well-known. For example, U.S. Pat. No. 5,994,679 shows a housing that comprises a base and a snap-on cover made of a material having a relatively low thermal coefficient of expansion. Within the housing is a layer of fibrous material that is relatively immune to temperature changes. A pair of support members of the same material, but hardened, support a thermal bed, which comprises a substantially U-shaped aluminum member, the legs of which define a slot for receiving the AWG or other planar lightguide circuit. The slot is filled with a thermally conductive grease that suspends the AWG and allows it to float within the slot, substantially completely covered by the legs of the U-shaped bed. Thus, the AWG is in a stress free position in the slot. The thermal grease also increases the thermal conductivity between the thermal bed and the circuit and insures that the temperature is uniform over the entire circuit and that there are no hot spots. On the top surface of one or both legs of the U-shaped bed is a heater, a pair of resistive temperature devices for monitoring the temperature of the bed, and a temperature controller. Leads from the temperature controller pass through electrical lead through pins to the exterior of the housing to supply power to the heater.
One problem with the aforementioned housing is that it requires a relatively large number of components, thereby increasing the complexity and cost of its assembly. Moreover, the housing must be relatively thick to accommodate the U-shaped bed, which diminishes its attractiveness for space-limited applications, such as when the housing is to be mounted on a printed-circuit board.
Accordingly, it would be desirable to provide a housing for an optical component that maintains the component at a constant temperature and which is compact and simple to assemble.
In accordance with the present invention, a housing is provided for maintaining a planar lightguide circuit at a temperature within a predetermined temperature range independent of ambient temperature. The housing includes a planar heating arrangement supporting and in thermal contact with the planar lightguide circuit. Also included is a frame assembly having a first surface on which the planar heating arrangement is fixed. The frame assembly has at least one opening through which extends at least one optical fiber coupled to the planar lightguide circuit. An overmold, which is molded around the frame assembly, includes at least one strain relief member through which the optical fiber extends.
In accordance with one aspect of the invention, the planar heating arrangement includes a thermally conductive ceramic substrate and a resistive heating element disposed on a first side of the substrate. The planar lightguide circuit may be disposed on a second side of the substrate.
In accordance with another aspect of the invention, the ceramic substrate is formed from aluminum-nitride.
In accordance with yet another aspect of the invention, an elastometric thermal interface pad is provided, which has a first surface in contact with the planar heating arrangement and a second surface in contact with the planar lightguide circuit.
In accordance with another aspect of the invention, the planar lightguide circuit and the substrate have substantially similar temperature coefficient of expansions.
In accordance with another aspect of the invention, the frame assembly includes a frame member and base and cover members secured to one another in an air tight, water resistant manner.
In accordance with yet another aspect of the invention, the strain relief member is integrally formed with the overmold and is configured as a tapered collar surrounding the optical fiber extending therethrough.